In automatic transmissions and other fluid pressure control systems, solenoid force motors are sometimes used to develop a variable hydraulic pilot pressure for adjusting the operating point of a pressure regulator valve. In general, the force motor pressure varies in direct proportion to the average current supplied to the solenoid coil, and a system controller modulates the coil energization to achieve a desired coil current corresponding to the desired hydraulic pressure.
Experience has shown that the solenoid coil current cannot be reliably controlled with conventional open-loop and/or closed-loop control strategies, due to circuit variations (such as wire harness length) and the wide temperature variations of the control valve environment. Typically, the control valve is submersed in hydraulic fluid, the temperature of which can vary from -40.degree. C. to +150.degree. C., resulting in corresponding changes in the solenoid coil resistance. One approach, disclosed in the U.S. Pat. No. 4,975,628 to Lemieux, utilizes a closed-loop integral control, with integrator gain terms being scheduled as a function of the solenoid temperature. On one hand, the control valve temperature can be easily determined by simply measuring the hydraulic fluid temperature, but on the other hand, the solenoid resistance varies so greatly that it is difficult to accurately and dynamically schedule the control gains over the entire operating range of the valve. As a result, it is difficult to achieve fast response while avoiding overshoot and steady-state error.